As is known, in hydraulic systems in which the operating fluid is circulated by means of a pump, the pressure pulses generated by the pump are transmitted along the conduits and may give rise to undesired noise and vibration.
To reduce pulsation, conduits are currently used in which the path of the fluid is so devised as to produce reflected pressure waves which interfere with the incident waves; and by appropriately selecting the length of the wave paths, destructive interference of the waves is achieved to substantially eliminate pulsation at a given frequency.
In one known embodiment, the above conduits comprise an outer tube, and an inner tube extending a given length from an end fitting and defining an annular chamber with the outer tube. The fluid flows through the fitting into the inner tube, and, at the outlet of the inner tube, the incoming pressure waves interfere with the waves travelling along the annular chamber and reflected by the end wall of the chamber defined by the fitting. For interference to be destructive, the length of the annular chamber must be a quarter of the wavelength of the pressure waves.
With the above sizing of the inner tube, known conduits of the aforementioned type are "tuned" to a given frequency, whereas, in a real circuit, the pressure pulses are periodic but not purely sinusoidal with a well defined frequency, and may be divided into a number of frequency components comprising a fundamental or first harmonic component, and harmonic components of frequencies equal to multiples of the fundamental frequency. This therefore poses the problem of attenuating a number of components, typically the first two or three harmonics which present a much higher amplitude as compared with the harmonics of a higher order.
One way of doing this is to use a conduit comprising a number of elementary conduits of the above type in series with one another and each tuned to a respective frequency. Such a solution, however, results in an excessive total length of the conduit, and is therefore unsuitable for automotive applications in which size is a critical factor.